An angular speed sensor (a gyro instrument) is employed for an attitude control and a navigation system of a mobile object, such as an airplane and a vehicle, and this sensor detects an angular speed of the mobile object by detecting a Coriolis signal associated with the angular speed.
As for the navigation system, when a current position is detected for the mobile object such as a vehicle equipped with the navigation system, there is known a method of measuring a traveling direction and a travel distance of the mobile object from a predetermined position, in addition to a method of conducting measurement by using the GPS (Global Positioning System). It is also known that in this measuring method based on the traveling direction and the travel distance, the current position, the traveling direction, and speed variation of the mobile object are detected, by using an angular speed sensor and an acceleration sensor.
In the case above, when the direction and magnitude of the acceleration or the like are detected, a detection axis is set for each of the sensors as a reference. On the premise that this detection axis is parallel with the traveling direction (e.g., a road surface), each parameter of each of the sensors is set, and these sensors are installed and fixed within the navigation system. In general, when the navigation system is provided within a vehicle, it is installed and fixed in such a manner as being parallel with a horizontal plane (ground surface) in the vehicle.
Furthermore, in general, the navigation system is installed on a front panel surface such as a central console, for instance, within the vehicle. When the navigation system is installed on the front panel surface such as the central console, it is required to enhance visibility of a display surface provided on the navigation system, as well as enhancing operability of buttons and switches. Therefore, the display surface and the surface including these buttons and switches are configured in such a manner as having an angular position facing slightly upward with respect to the vertical direction.
Therefore, the navigation system is installed on the front panel surface in such a manner that the rear part of the navigation system (forward of the vehicle) is set to be lower position in the vertical direction. Accordingly, the navigation system is installed being inclined with respect to the front panel surface of the vehicle.
The mounting position of this navigation system on the front panel surface varies depending on the vehicle types, options set by a user, or the like, and various positions are possible such as upper side or lower side of the central console. Therefore, depending on where the navigation system is installed on the front panel surface, various mounting angles (the angles with respect to the horizontal direction) of the navigation system are possible. For example, if the navigation system is provided on the lower position on the central console, it is required to improve the visibility, and therefore, the navigation system should be upward leaning much more than the case where it is provided on the upper position on the front panel surface.
When the navigation system is installed on the vehicle at a certain angle as described above, there is a possibility that the angle exceeds an allowable value which is preset in the angular speed sensor.
On such an occasion above, the direction of the detection axis that is preset in each of the sensors in the navigation system may be largely different from the direction (horizontal direction) of the acceleration or the like, which is applied on each of the sensors when detection is actually performed. Therefore, errors such as an offset of sensitivity of the sensor itself may be included, and thereby causing a problem that positioning accuracy as to the vehicle current position and measurement accuracy as to the speed are deteriorated.
By way of example, patent document 1 discusses this subject above and suggests a configuration to solve the problem. FIG. 18 is a sectional view illustrating the central console on which the navigation system is mounted. According to the description in the document, the navigation system 100 is inclined with respect to the horizontal direction from the state before mounted on the central console 110 (FIG. 18(A)), so that the display surface 102 is installed facing upwardly along with the inclination of the central console 110. Then, the detection axis 103 of the acceleration sensor 101 is also inclined by θ with respect to the horizontal direction, in accordance with the mounting angle θ of the navigation system 100 (FIG. 18(B) and FIG. 18(C)).
If it is assumed that the vehicle drives forward (rightward in FIG. 18), the acceleration “Ax” is applied to the navigation system in the direction that is opposite to the forward movement. In addition, the acceleration “Az” is applied to the acceleration sensor 101 in the vertical direction, which is caused by the vehicle vibration and an impactive force from the ground.
As shown in FIG. 18(A), when the navigation system is in the state being horizontal, the acceleration sensor 101 receives the horizontal acceleration Ax and the vertical acceleration Az as the accelerations in the respective directions. However, as shown in FIG. 18(B) and FIG. 18(C), when the navigation system in the state being inclined, the acceleration sensor 101 detects the horizontal acceleration, as the vector sum At (=Gx+Gz=cos θ×Ax+sin θ×Az) according to the inclination, based on the horizontal acceleration Ax and the vertical acceleration Az.
Since the resultant acceleration “At” is different from the acceleration “Ax” that is to be actually detected, this error may cause a deterioration of positioning accuracy or the like, as to the current position of the vehicle. As the mounting angle θ of the navigation system 100 increases, the component of the acceleration “Ax” of the vehicle in the traveling direction is more deteriorated and influence from the vertical acceleration “Az” becomes more remarkable, thus leading to a noticeable error.
As for the angular speed sensor, similar to the above acceleration sensor, an error may occur due to the mounting angle onto the vehicle, thereby causing deterioration in the positioning accuracy.
If this error is corrected utilizing software, there is a problem that sufficient compensation cannot be obtained in the initial state, and there is also a problem that when the mounting angle is large, it is difficult to carry out the compensation. In addition, there is another problem in manufacturing cost and the like. Therefore, the patent document discloses a configuration in which a mounting slit is formed at a mounting angle being inclined, in a mounting member which fixedly supports a sensing unit, and the sensing unit is engaged with the mounting slit.
Patent document 1: Japanese Unexamined Patent Application Publication No. 2003-227844